Analog signal processing is an important part of many modern communications systems, such as satellite systems, for example. A received signal from an antenna may include digital or analog information, and it may ultimately be processed digitally, but unless the signal can be digitized directly (a challenging prospect as the frequency of the signal increases), there may be some amount of analog signal processing required. This may include amplification, filtering, transmission over some distance, distribution to multiple receivers/transmitters, and frequency conversion for up- or down-conversion. RF and microwave components are very mature, and a baseline level of performance has been demonstrated for these processing functions. Demand for capacity and the broader use and congestion of the electromagnetic spectrum are among the forces increasing the complexity, cost, and performance requirements of analog systems. As higher levels of performance and higher carrier frequencies become desired, especially in the millimeter wave portion of the spectrum, new approaches may be desirable to meet the challenges. Photonics offers certain advantages in this regard: bandwidth; size, weight and power (SWaP); linearity; frequency agility; and providing a reconfigurable infrastructure for analog signal processing.
Photonic systems may cover a wide frequency range and instantaneous bandwidth (IBW), with frequency ranges extending to millimeter waves and an IBW as large as 4 GHz or more. Optical fiber provides an exceptionally low loss transmission medium, with roughly 0.2 dB/km loss regardless of the analog frequency it is carrying. Wavelength division multiplexing may further extend bandwidth by allowing multiple signals to share the same path.
The SWaP of a photonic system may be relatively low due in part to the wide bandwidth of the system: a single set of hardware may cover many decades of the RF spectrum. Optical fiber is also substantially lighter in weight than coaxial cable, and its inherent immunity to electromagnetic interference reduces the cost, effort and space desired for shielding.
As with RF signal processing, it may be desired in some applications to perform filtering in the photonic domain as well. One approach to filtering in the photonic domain involves ring waveguides or resonators. An optical ring resonator is a closed loop waveguide coupled to an optical input and output. As a result of constructive interference within the ring, only certain frequencies of light will be at resonance within the ring and therefore pass to the output. As a result, the optical ring resonator acts as a filter for these wavelengths of light.
In some configurations, more than one ring resonator may be connected in series. An example of such a configuration is set forth in U.S. Pat. No. 6,411,752 to Little et al. This patent discloses a device in which optical resonators are vertically coupled on top of bus waveguides, and are separated from the waveguides by a buffer layer of arbitrary thickness.
Despite the existence of such configurations, further enhancements may be desirable with respect to optical filters to provide a desired filter response in certain applications.